The present invention relates generally to voltage controlled variable gain attenuators and amplifiers, and more particularly to a voltage controlled variable gain attenuator and amplifier having a gain curve that is nearly independent of the effects of process variation and temperature variation.
Present medical ultrasound and industrial imaging systems employ as many as 256 signal processing channels to create a clear image. Advances in the technology associated with signal processing in this area indicate that the channel count may be higher in future systems. A typical ultrasound channel consists of a piezo-electric transducer that supplies a signal to a variable gain amplifier which in turn drives an analog to digital converter. The piezo-electric transducer is excited by a high voltage transmitter which creates a signal that is applied (for example) to a patient or to industrial material undergoing an imaging examination. The output of the analog to digital converter typically is further processed by digital circuitry and software for purposes of displaying and analyzing the image in question. Future ultrasound systems are expected to make increasing demands on the precision of the various system components to create a more precise image. One requirement for future systems is that the gain of each channel match the gains of the other channels more accurately than is the case for present systems. This is desirable because the lack of precise channel matching creates image “artifacts” which have the effect of degrading the quality of the displayed image. In effect, the image artifacts caused by the lack of precise channel matching may be erroneously interpreted as meaningful signals by an image processor.
Ultrasound signal systems employ time-dependent gain control to adjust the system gain in order to prevent system overloading. This is commonly achieved through the use of a voltage controlled variable gain amplifier. At the beginning of a sweep a gain control voltage sets the amplifier gain at its lowest level and from that point the gain is gradually increased so that weak signals can be properly amplified. (In ultrasound signal systems, a sweep or scan is the period of time between the shallowest echo and the deepest echo.) The system gain versus control characteristic increases as a function of time, and usually the gain, expressed in dB (decibels), is linearly related to the gain control voltage.
Unfortunately, one of the most unpredictable elements in this signal processing progression is the voltage controlled variable gain amplifier. Typically, to achieve a linear gain expressed in dB, a piece-wise approximation to an ideal curve has been provided. See commonly assigned. U.S. Pat. No. 6,229,375 entitled “Programmable Low Noise CMOS Differentially Voltage Controlled Logarithmic Attenuator and Method” issued May 8, 2001 to the present inventor. This technique achieves only ±1 dB gain precision, and typically is complex and costly. The closest prior art is believed to include the above mentioned U.S. Pat. No. 6,229,375 and also U.S. Pat. Nos. 5,880,618 and 5,077,541 which indicate the present state of the art in the area of voltage controlled variable gain amplifiers having gain that is linear in dB (i.e., gain that is linear when expressed in decibels). In instrumentation applications it is very common to express gain or attenuation (i.e., gain having a value of less than 1) in dB, and it also is very common to control the gain or attenuation in dB in response to a control voltage.
The use of voltage-controlled FET resistors (i.e., field effect transistors used as resistors) in various applications, including attenuators, is known. However, the precision of the resistance of FET resistors is very poor.
Band gap reference voltage circuits which produce a voltage that is proportional to absolute temperature and essentially independent of process parameter variations are well-known in the art. FIG. 6 herein shows such a bandgap reference voltage circuit
Thus, there is an unmet need for a high precision, voltage-controlled (or current-controlled) variable gain circuit element which can be utilized to provide ripple-free control of attenuation, gain, or other circuit or system parameters without resorting to piecewise-linear approximation techniques.
There also is an unmet need for a high precision, voltage-controlled (or current-controlled) variable gain circuit element which can be utilized to provide ripple-free control of attenuation, gain, or other circuit or system parameters without resorting to piecewise-linear approximation techniques, and which is substantially independent of the effects of integrated circuit process parameter variations and/or temperature variations.
There also is an unmet need for a voltage-controlled variable gain amplifier which is more accurate than the prior art and is independent of the effects of process variation and temperature variation.
There also is an unmet need for a voltage controlled attenuator which is more accurate than the prior art and is independent of process variation and temperature variation and is suitable for use in the gain control portion of a voltage controlled variable gain amplifier.